Magnetic filters have been used for many years for separating strongly magnetisable particles, for example ferromagnetic particles, from a liquid. Such magnetic filters are described in U.S. Pat. No. 3,326,374, British patent specification No. 1,059,635 and British patent specification No. 1,204,324. More recently, however, much interest has been shown in apparatus for separating more weakly magnetisable particles, for example paramagnetic particles, from a mixture of solids and liquids, for example a clay slurry. In German Offenlegungsschrift No. 24 33 008, there is described such apparatus for separating magnetisable particles from a fluid in which they are suspended, which apparatus comprises one or more separating chambers movable into, and out of, a first zone and a magnet, possibly a superconducting magnet, which is intended to establish a continuous magnetic field in the first zone when the apparatus is in use. Each separating chamber comprises a canister provided with an inlet for feed slurry (which comprises magnetisable particles in suspension in a fluid) and an outlet for treated slurry, and a liquid-permeable packing of magnetisable material of approximately uniform density and approximately uniform cross-sectional area disposed within the cannister between the inlet and the outlet. The packing material may be paramagnetic or ferromagnetic and may be in particulate or filamentary form or even in the form of a foam-like material. For example the packing material may be constituted by ferromagnetic spherules, pellets or more irregularly shaped particles of ferromagnetic material, such as filings or chippings; or ferromagnetic wool, such as steel wool; or ferromagnetic wire mesh; or ferromagnetic wires or filaments packed individually or in bundles.
When a suitable feed slurry is passed through a separating chamber containing packing material in one of the forms described above, the separating chamber being positioned in the first zone in which a magnetic field is established, the magnetisable particles in the slurry are magnetised and captured in the packing material. When the quantity of magnetisable particles in the treated slurry leaving the outlet of the separating chamber reaches an unacceptably high level, the flow of feed slurry through the separating chamber is stopped and the separating chamber is moved to a second zone, out of the influence of the magnetic field, where the magnetisable particles captured in the packing material are removed, for example by flushing the separating chamber with water at high pressure. If the apparatus comprises two separating chambers, feed slurry may be passed through one separating chamber in the first zone whilst magnetisable particles are being removed from the other separating chamber in the second zone, the positions of the separating chambers subsequently being reversed. In this way feed slurry may be supplied to the apparatus continuously, except when the separating chambers are actually being moved.
In order that as large a proportion as possible of the separation cycle is spent productively, that is in actually treating slurry, the length of time for which the feed slurry is passed through each separating chamber should be long in comparison to the length of time which it takes to reverse the positions of the separating chambers. During the former time as large a quantity of feed slurry as possible should be passed through the separating chamber before it is necessary to regenerate the packing material. However, in practice, the slurry will contain magnetisable particles of different sizes and different magnetic susceptibilities. Thus the magnetisable particles will not be captured evenly throughout the packing material. In fact, when the feed slurry first enters the separating chamber, the magnetisable particles are initially captured mainly in the first part of the packing material encountered by the slurry. When this part of the packing material is substantially completely filled, those parts of the packing material further downstream are progressively filled. However, those magnetisable particles which are difficult to capture, that is the small and/or weakly magnetisable particles, tend to pass some way through the packing material before they are captured. A proportion of the magnetisable particles will even pass completely through the packing material without being captured. It is therefore advantageous if the packing material is as long as possible consistent with the dimensions of the magnetic field. However, as the packing material begins to fill up with magnetisable material in the upstream regions, the proportion of magnetisable particles passing completely through the packing material will increase. When this proportion has increased to an unacceptably high level, the packing material will require regeneration. However, only those collecting sites in the upstream regions of the packing material will have been substantially completely filled. In order that as large a quantity of feed slurry as possible may be passed through the separating chamber, it is advantageous for the cross-section of the packing material transverse to the direction of flow of the slurry to be as large as possible. However, this dimension is again limited by the dimensions of the magnetic field. Furthermore, the probability of a particular magnetisable particle being captured in the packing material is approximately inversely proportional to the linear velocity of the slurry through the packing material, other factors being equal. Therefore the rate at which the feed slurry is passed through the separating chamber may not be increased above a certain value if the capture of the small and/or more weakly magnetisable particles is not to suffer. It may therefore be seen that the quantity of feed slurry passed through each separating chamber during each cycle is limited by a number of factors.